Morphological classification of soft-shell clam, Mya arenaria, hemocytes: Phenotypic description of hemic neoplasia

In Atlantic Canada, hemic neoplasia (HN) has a negative impact on aquaculture and fisheries of the soft-shell clam, Mya arenaria. HN is a cellular disorder that morphologically and functionally alters the hemocytes. These abnormal hemocytes are found proliferating in the hemolymph and infiltrating tissues. Cells described as neoplastic are larger with a high nucleus to cytoplasm ratio compared to normal hemocytes. Although the morphology and behavior of neoplastic cells has been described, information regarding the characteristics of normal hemocytes in the soft-shell clam is lacking. The present study used cytology and transmission electron microscopy to characterize 5 types of hemocytes in clams. Hemolymph samples of normal clams had a mean differential hemocyte count of 62.6% agranular and 34.7% granular cells. Evaluation of the normal hemocytes provided descriptive information that was then useful for recognizing neoplastic cells. Two types of neoplastic cells and intermediate forms were observed at various levels of disease intensity. This study proposes criteria for recognizing neoplastic cells.

Flow cytometry is a technique to differentiate neoplastic from non-neoplastic cells in a non-lethal, labor efficient and reliable manner. Using the flow cytometer case definition for an individual clam positive for HN, clams from six populations in Atlantic Canada were statistically compared. Cytology was used for confirmatory purposes. Developing a case definition for a HN positive population was more challenging. Disease prevalence as well as intensity was considered. A sample population from Saguenay Bay, QC was considered negative because all clams were classified as negative on an individual basis. A sample population from North River, PEI had 35% prevalence and was the only population with high disease intensity; it was considered population positive for HN. The four other populations also had high prevalence but low intensity and were not able to be classified as disease positive or negative. The status of these populations should be checked on a seasonal basis to see if prevalence and intensity change.